1. Field of the Invention
The present invention relates to a process for the dissolution of copper metal. More particularly, the invention relates to a process for producing a copper-containing aqueous solution by dissolving copper in the presence of an oxidant in an aqueous leach liquor containing monoethanolamine and (monoethanolammonium)2 carbonate (HMEA)2CO3.
2. Description of the Prior Art
It is known to employ copper-containing aqueous solutions as biocidal fluids, for example, for the pressure treatment of lumber and for water purification. Examples of such fluids and uses thereof may be found, for example, in U.S. Pat. No. 4,929,454 and U.S. Pat. No. 6,294,071. The copper-containing solutions may be formulated, for example, by dissolving copper in aqueous solutions containing alkyl amines or alkyl hydroxy amines, such as 2-hydroxyethylamine.
Alternatively, copper-containing solutions may be produced by reacting copper oxide with chromic acid and arsenous acid to produce a solution of the copper with chrome and arsenic. The solution is subsequently diluted with water and the resulting aqueous solution may be injected into wood under pressure.
This chromated copper arsenate (“CCA”) is the primary additive used in the treatment of wood against termite and other biological infestation. Although the CCA is very effective, it has come under increased pressure because of the environmental concerns associated with chromium and arsenic.
A new generation of pesticide is now emerging that appears to be efficacious, and which relies on the use of copper (in larger quantities than in the CCA) in combination with other pesticidal components, such as quaternary amines and triazoles. The copper is typically applied as a solution of the monoethanolamine complex of copper carbonate or borate. The commercial form of the copper concentrate usually contains about 100 to 130 g/l copper which is diluted with water prior to injection into the wood.
The copper complex is typically produced commercially by the dissolution of basic copper carbonate in a solution of monoethanolamine (MEA), followed by further carbonation or addition of boric acid. The reactions can be approximately represented by the following equations:CuCO3Cu(OH)2+7MEA→Cu(MEA)3.5CO3+Cu(MEA)3.5(OH)2Cu(MEA)3.5(OH)2+CO2(or Boric acid)→Cu(MEA)3.5CO3+H2O
The production of the copper carbonate precursor has its own production and raw material costs, and a brine waste is generated which gives rise to environmental concerns. A more efficient process might be to produce the complex without the aid of an isolated precursor. It is known from the prior art that ammonia and carbon dioxide in water can be used to dissolve copper metal with oxygen from air as the oxidant. This is represented by the following equation:Cu+2NH3+(NH4)2CO3+1/2O2→Cu(NH3)4CO3+H2O
The reaction proceeds well and has been the basis for copper dissolution in several commercial facilities. However, if the ammonia is not initially carbonated, the kinetics are very poor which makes the process unattractive from a commercial standpoint.
A need exists for a more efficient process for producing copper-containing aqueous solutions, suitable for use in the wood-treatment industry. The present invention seeks to fill that need.